/Engine/Source/Developer/TextureFormat/Public/ChildTextureFormat.h
Make the child type think about if they need a key string or not, by making it pure virtual. InMipCount and InMip0Dimensions are only valid for non-virtual textures (VTs should never call this function as they never tile)
[FString](API\Runtime\Core\Containers\FString) GetChildDerivedDataKeyString ( const [FTextureBuildSettings](API\Developer\TextureCompressor\FTextureBuildSettings) & InBuildSettings, int32 InMipCount, const FIntVector3 & InMip0Dimensions ) const